1. Field of the Invention
The present invention relates to a diaphragm-control apparatus for a camera.
2. Related Background Art
A typical example of a conventional diaphragm-control system is described in U.S. Pat. No. 4,307,948. According to this system, changes in light metering value output upon changing of an aperture value are detected. A diaphragm or aperture stopping signal is generated to interrupt stopping-down of the aperture when the measured light metering value reaches a desired aperture value.
A conventional diaphragm-control apparatus shown in FIG. 1 will be described below. Light passing through a photographing lens is photoelectrically converted by a photodiode PD as a light metering sensor in the camera body. A current from the photodiode PD is converted into a voltage logarithmically compressed by a negative feedback diode D of a light metering amplifier OP1. An output V.alpha. from the light metering amplifier OP1 is decreased when the size of an aperture or stop St is stopped down. When the output V.alpha. is compared with a voltage Vref corresponding to the desired aperture value and reaches the voltage Vref, the aperture stopping signal is output to interrupt stopping-down of the apertures St. However, a delay time dt is required until a stopping mechanism is actually operated after the aperture stopping signal is output. For this reason, the light metering output V.alpha. is differentiated by a differentiator constituted by an amplifier OP2, a resistor R, and a capacitor C. An advance amount of the aperture value within the delay time is added to the light metering output V.alpha. to obtain an output V.beta.. This output is input to a comparator COM. The relationship between the inputs and the output with respect to the amplifier OP2 is given as follows: EQU V.beta.(t)={1+(CRd/dt)}V.alpha.(t)
If the advance amount of the aperture value within the delay time dt is converted into a value dV corresponding to the light metering output, the value dV is given by: EQU dV=CR(d/dt)V.alpha.(t)
If the capacitance and the resistance are selected to satisfy the above equation, the change dV in light metering value within the delay time is taken into account for diaphragm control.
An output from the comparator COM serves as an aperture stopping signal to control a transistor Tr and therefore an aperture stopping magnet Mg. As a result, stopping-down of the aperture St is interrupted.
FIG. 2 is a graph showing that the output V.alpha. of the amplifier OP1 and the output V.beta. of the amplifier OP2 are reduced according to changes in stopping-down of the aperture St. As is apparent from FIG. 2, the output V.beta. is decreased faster than the output V.alpha. by a value obtained by multiplying a differential of the output V.alpha. with a coefficient CR. More specifically, the output V.beta. is a predictor of the light metering value. If the output V.beta. is compared with the voltage Vref corresponding to the desired aperture value, the delay time dt is taken into consideration in diaphragm control.
This conventional diaphragm-control apparatus employs analog control and requires a large number of components in practice, as shown in FIG. 1. If the diaphragm-control apparatus is integrated as an IC, the number of pins is large. Therefore, it is very cumbersome to mount such an IC on a board. In addition, since the apparatus includes a differentiator, it tends to be adversely affected, thus requiring shielding implementation in actual mounting. Furthermore, since a differential coefficient of the light metering output is used to estimate the advance amount of the aperture value within the delay time, good prediction precision can be achieved for a linear aperture travelling velocity, i.e., a predetermined gradient of the light metering output V.alpha.. However, if the aperture is stopped down exponentially, the gradient is changed within the delay time. In this case, the longer the delay time becomes, the smaller the predictor for the advance amount of aperture value within the delay time bcomes. As a result, the resultant aperture value is larger than the desired aperture value.